Ansible failed while update rubygems

Hi everybody!
I need help to install edx-platform by link Open edX Step-By-Step Production Installation Guide - Blog

I’ve got an error in log file:
TASK [rbenv : update rubygems] *************************************************
fatal: [localhost]: FAILED! => {“changed”: true, “cmd”: “gem update --system 2.7.8”, “delta”: “0:00:00.005553”, “end”: “2021-05-28 12:48:13.882957”, “msg”: “non-zero return code”, “rc”: 127, “start”: “2021-05-28 12:48:13.877404”, “stderr”: “/bin/sh: 1: gem: not found”, “stderr_lines”: ["/bin/sh: 1: gem: not found"], “stdout”: “”, “stdout_lines”: }

PLAY RECAP *********************************************************************
localhost : ok=489 changed=168 unreachable=0 failed=1 skipped=1073 rescued=0 ignored=0

============================================================
Ansible failed!

Decoded error:
== cmd ===========================
gem update --system 2.7.8
== msg ===========================
non-zero return code
== stderr ===========================
/bin/sh: 1: gem: not found

============================================================
Installation failed!

If you need help, see https://open.edx.org/getting-help .
When asking for help, please provide as much information as you can.
These might be helpful:
Your log file is at /home/dtsmyh@scnsoft.com/logs/install-20210528-120031.log
Your environment:
OPENEDX_RELEASE=open-release/koa.master

Installation finished at 2021-05-28 12:48:14

Any ideas?